1. Field of the Invention
This invention relates to the molybdenum-catalyzed epoxidation of C.sub.3 to C.sub.20 olefins with tertiary butyl hydroperoxide or tertiary amyl hydroperoxide in liquid phase in a polar reaction medium.
2. Prior Art
The epoxidation of olefins to give various oxide compounds has long been an area of study by those skilled in the art. It is well known that the reactivities of the various olefins differs with the number of substituents on the carbon atoms involved in the double bond. Ethylene itself has the lowest relative rate of epoxidation, with propylene and other alpha olefins being the next slowest. Compounds of the formula R.sub.2 C.dbd.CR.sub.2 where R simply represents alkyl or other substituents may be epoxidized fastest.
Of course, the production of ethylene oxide from ethylene has long been known to be accomplished by reaction with molecular oxygen over a silver catalyst. Numerous patents have issued on various silver-catalyzed processes for the production of ethylene oxide.
Unfortunately, the silver catalyst route is poor for olefins other than ethylene. For a long time the commercial production of propylene oxide could only be accomplished via the cumbersome chlorohydrin process.
Another commercial process for the manufacture of substituted oxides from alpha olefins such as propylene was not discovered until U.S. Pat. No. 3,351,635 taught that an organic oxide compound could be made by reacting an olefinically unsaturated compound with an organic hydroperoxide in the presence of a molybdenum, tungsten, titanium, columbium, tantalum, rhenium, selenium, chromium, zirconium, tellurium or uranium catalyst. U.S. Pat. No. 3,350,422 teaches a similar process using a soluble vanadium catalyst. Molybdenum is the preferred catalyst. A substantial excess of olefin relative to the hydroperoxide is taught as the normal procedure for the reaction. See also U.S. Pat. No. 3,526,645 which teaches the slow addition of organic hydroperoxide to an excess of olefin as preferred.
However, even though this work was recognized as extremely important in the development of a commercial propylene oxide process that did not depend on the chlorohydrin route, it has been recognized that the molybdenum process has a number of problems. For example, large quantities of the alcohol corresponding to the peroxide used were formed; if t-butyl hydroperoxide was used as a co-reactant, then a use or market for t-butyl alcohol is required. With propylene, various undesirable propylene dimers, sometimes called hexenes, are formed. Besides being undesirable in that propylene is consumed, problems are caused in separating the desired propylene oxide from the product mix. In addition, the molybdenum catalyst may not be stable or the recovery of the catalyst for recycle may be poor.
A number of other methods for the production of alkylene oxides from epoxidizing olefins (particularly propylene) have been proposed. U.S. Pat. No. 3,666,777 to Sargenti reveals a process for epoxidizing propylene using a molybdenum-containing epoxidation catalyst solution prepared by heating molybdenum powder with a stream containing unreacted tertiary butyl hydroperoxide used in the epoxidation process as the oxidizing agent and polyhydric compounds. The polyhydric compounds are to have a molecular weight from 200 to 300 and are to be formed as a by-product in the epoxidation process. A process for preparing propylene oxide by direct oxidation of propylene with an organic hydroperoxide in the presence of a catalyst (such as molybdenum or vanadium) is described in British Patent No. 1,338,015 to Atlantic-Richfield. The improvement therein resides in the inclusion of a free radical inhibitor in the reaction mixture to help eliminate the formation of C.sub.5 to C.sub.7 hydrocarbon by-products which must be removed by extractive distillation. Proposed free radical inhibitors are tertiary butyl catechol and 2,6-di-t-butyl-4-methyl phenol.
Stein, et al. in U.S. Pat. No. 3,849,451 have improved upon the Kollar process of U.S. Pat. Nos. 3,350,422 and 3,351,635 by requiring a close control of the reaction temperature, between 90-200.degree. C. and autogeneous pressures, among other parameters. Stein et al. also suggest the use of several reaction vessels with somewhat higher temperatures in the last zones to insure more complete reaction. The primary benefits seem to be improved yields and reduced side reactions. Prescher et al. in U.S. Reissue Pat. No. Re.31,381 disclose a process for the preparation of propylene oxide from propylene and hydrogen peroxide wherein plural reactors such as stirred kettles, tubular reactors and loop reactors may be used. They recommend, as an example, the use of a train of several stirred kettles, such as a cascade of 3 to 6 kettle reactors or the use of 1 to 3 stirred kettles arranged in series followed by a tubular reactor.
Russell U.S. Pat. No. 3,418,430 discloses a process for producing propylene oxide by reacting propylene with an organic hydroperoxide in solvent solution in the presence of a metallic epoxidation catalyst, such as a compound of molybdenum at a mole ratio of propylene to hydroperoxide of 0.5:1 to 100:1 (preferably 2:1 to 10:1) at a temperature of -20.degree. to 200.degree. C. (preferably 50-120.degree. C.) and a pressure of about atmospheric to 1000 psia, with a low olefin conversion per pass (e.g., 10-30%) wherein unreacted oxygen is removed from the unreacted propylene.
Sheng et al. U.S. Pat. No. 3,434,975 discloses a method for making molybdenum compounds useful to catalyze the reaction of olefins with organic hydroperoxides wherein metallic molybdenum is reacted with an organic hydroperoxide, such as tertiary butyl hydroperoxide, a peracid or hydrogen peroxide in the presence of a saturated C.sub.1 -C.sub.4 alcohol.
The molybdenum-catalyzed epoxidation of alpha olefins and alpha substituted olefins with relatively less stable hydroperoxides may be accomplished according to U.S. Pat. No. 3,862,961 to Sheng, et al. by employing a critical amount of a stabilizing agent consisting of a C.sub.3 to C.sub.9 secondary or tertiary monohydric alcohol. The preferred alcohol seems to be tertiary butyl alcohol. Citric acid is used to minimize the iron-catalyzed decomposition of the organic hydroperoxide without adversely affecting the reaction between the hydroperoxide and the olefin in a similar oxirane producing process taught by Herzog in U.S. Pat. No. 3,928,393. The inventors in U.S. Pat. No. 4,217,287 discovered that if barium oxide is present in the reaction mixture, the catalytic epoxidation of olefins with organic hydroperoxides can be successfully carried out with good selectivity to the epoxide based on hydroperoxide converted when a relatively low olefin to hydroperoxide mole ratio is used. The alpha-olefinically unsaturated compound must be added incrementally to the organic hydroperoxide to provide an excess of hydroperoxide that is effective.
Selective epoxidation of olefins with cumene hydroperoxide (CHP) can be accomplished at high CHP to olefin ratios if barium oxide is present with the molybdenum catalyst as reported by Wu and Swift in "Selective Olefin Epoxidation at High Hydroperoxide to Olefin Ratios," Journal of Catalysis, Vol. 43, 380-383 (1976).
Catalysts other than molybdenum have been tried. Copper polyphthalocyanine which has been activated by contact with an aromatic heterocyclic amine is an effective catalyst for the oxidation of certain aliphatic and alicyclic compounds (propylene, for instance) as discovered by Brownstein, et al. described in U.S. Pat. No. 4,028,423.
Various methods for preparing molybdenum catalysts useful in these olefin epoxidation methods are described in the following patents: U.S. Pat. No. 3,362,972 to Kollar; U.S. Pat. No. 3,480,563 to Bonetti, et al.; U.S. Pat. No. 3,578,690 to Becker; U.S. Pat. No. 3,953,362 and U. S. Pat. No. 4,009,122 both to Lines, et al.
It has also been proposed to use the tertiary butyl alcohol that is formed when propylene is reacted with tertiary butyl hydroperoxide as an intermediate in the synthesis of another organic compound. Thus, Schneider, in U.S. Pat. No. 3,801,667, proposes a method for the preparation of isoprene wherein, as the second step of a six step process, tertiarybutyl hydroperoxide is reacted with propylene in accordance with U.S. Pat. No. 3,418,340 to provide tertiary butyl alcohol. Connor et al. in U.S. Pat. No. 3,836,603 propose to use the tertiary butyl alcohol as an intermediate in a multi-step process for the manufacture of p-xylene.
Also pertinent to the subject discovery are those patents which address schemes for separating propylene oxide from the other by-products produced. These patents demonstrate a high concern for separating out the useful propylene oxide from the close boiling hexene oligomers. It would be a great progression in the art if a method could be devised where the oligomer by-products would be produced not at all or in such low proportions that a separate separation step would not be necessary as in these patents.
U.S. Pat. No. 3,464,897 addresses the separation of propylene oxide from other hydrocarbons having boiling points close to propylene oxide by distilling the mixture in the presence of an open chain or cyclic paraffin containing from 8 to 12 carbon atoms. Similarly, propylene oxide can be separated from water using identical entrainers as disclosed in U.S. Pat. No. 3,607,669. Propylene oxide is purified from its by-products by fractionation in the presence of a hydrocarbon having from 8 to 20 carbon atoms according to U.S. Pat. No. 3,843,488. Additionally, U.S. Pat. No. 3,909,366 teaches that propylene oxide may be purified with respect to contaminating paraffinic and olefinic hydocarbons by extractive distillation in the presence of an aromatic hydrocarbon having from 6 to 12 carbon atoms.